And you can think of the kernel as a "driver in a car". Main concerns for a kernel and what it does is how it performs on the device. Controls speed of the device which in turn effects battery life.
Each phone is a little different and we all use our phones differently. So a kernel and ROM combo that works great for me, may not work so well on your phone. You can mix and match pretty much any kernel/ROM and many of the ROM devs already have a custom kernel loaded in once you flash a new ROM.